(a) Field of the Invention
The present invention relates to a display device and a driving method thereof.
(b) Description of Related Art
Recent trends of light-weighted and thin personal computers and televisions sets also require light-weighted and thin display devices, and flat panel displays satisfying such a requirement is being substituted for conventional cathode ray tubes (CRT).
The flat panel displays include a liquid crystal display (LCD), field emission display (FED), organic light emitting display (OLED), plasma display panel (PDP), and so on.
Generally, an active matrix flat panel display includes a plurality of pixels arranged in a matrix and displays images by controlling the luminance of the pixels based on given luminance information. An OLED displays image by electrically exciting light emitting organic material, and it is a self-emissive display device having low power consumption, wide viewing angle, and fast response time, thereby being advantageous for displaying motion images.
A pixel of an OLED includes a light emitting element and a driving thin film transistor (TFT). The TFT includes polysilicon or amorphous silicon. A polysilicon TFT has several advantages, but it also has disadvantages such as the complexity of manufacturing polysilicon, thereby increasing the manufacturing cost. In addition, it is hard to make an OLED employing polysilicon TFTs be large.
On the contrary, an amorphous silicon TFT is able to make a large OLED and to facilitate the manufacturing process. However, an OLED employing amorphous silicon TFTs exhibit a deterioration of bias stress stability that an output current of the TFT is reduced as time goes under a long-time application of high DC control voltages with high driving voltages such that the luminance is varied for a given data voltage. In addition, since the driving transistor and the light emitting element are connected to each other, a node voltage varies depending on the data voltage, which should be substantially constant. That is, the node voltage increases as the data voltage increases and thus a range of control voltages of the driving transistor is small compared with the data voltage. Accordingly, the data voltages are not efficiently used.